1. Field of the Invention
The present invention relates to a method for separating nickel, cobalt, and rare earths from positive and negative electrode active materials of nickel-metal hydride batteries which are spent ones or scraps generated during a manufacturing process.
2. Description of the Related Art
In recent years, environmental problems, such as acid rain attributable to acid gas of nitrogen oxide, sulfur oxide, etc. which is emitted into the air, and global warming due to carbon dioxide gas, have been highlighted as global problems. In order to reduce one of the causes, that is, contamination by exhaust gas of automobiles, hybrid vehicles equipped with a secondary battery such as a nickel-metal hydride battery have been attracting attention.
A nickel-metal hydride battery comprises a positive electrode, a negative electrode, an electrode terminal, and an electrolyte, as functional parts, and furthermore comprises such as an electrode substrate, a separator disposed between positive and negative electrodes, a case for housing these parts, as structural parts.
Various materials and components are comprised in such a manner that the positive electrode active material is composed of nickel hydroxide containing additional trace elements; the negative electrode active material is composed of a hydrogen storage alloy containing nickel, cobalt, rare earth elements (misch metal), etc.; the electrode substrate is composed of a nickel plate or a foam Ni plate, a nickel-plated iron plate, etc.; the separator is composed of synthetic resin; the electrolyte comprises a potassium hydroxide solution; the electrode terminal material is composed of metal of copper, iron, etc.; and the case is composed of synthetic resin, steel, etc.
As a structure of the nickel-metal hydride battery, first, there is an electrode body configured such that positive electrodes and negative electrodes are alternately layered, with disposing synthetic resin between each of the electrodes as a separator. The electrode body is placed into a case made of synthetic resin or steel, then the electrodes and the case are connected with electrode terminal materials made of steel or iron metal, and finally an electrolyte containing a potassium hydroxide solution as a main component is filled between the electrodes, and the case is sealed.
A large-capacity nickel-metal hydride battery installed in hybrid vehicles is replaced with a new one when deteriorating with use, or removed when a vehicle is scrapped, and then discarded as a spent nickel-metal hydride battery. In addition, there are generated defective products resulting during a manufacturing process of nickel-metal hydride batteries; parts, such as an active material and a negative or positive electrode material, which is not assembled into a battery and thus becomes unnecessary; and further trial products. As mentioned above, the spent nickel-metal hydride battery, defective products, parts, and trial products, etc. (hereinafter, these defective products, parts, trial products, etc. are also collectively referred to as a nickel-metal hydride battery) contain many kinds of rare and valuable metals, such as nickel, cobalt, and rare earth elements, and therefore recovering and reusing these valuable metals have been attempted.
However, nickel-metal hydride batteries have a complex and solid structure, and furthermore, are comprised of materials many of which are chemically stable. In addition, if a spent battery is disassembled without preparation, an abnormality, such as heat generation and ignition caused by partial short-circuiting inside the battery, could arise, and therefore, at the time of disassembling, a deactivation treatment, such as discharging and removing residual electric charge of the battery, is needed in advance. Accordingly, it was not easy that metals contained in a spent nickel-metal hydride battery, such as nickel, cobalt, and rare earth elements, were separated and recovered to be reused as materials for new batteries.
Therefore, as a method for recycling a metal from a spent nickel-metal hydride battery, there has been actually performed in recent years, for example, a method wherein a spent nickel-metal hydride battery is placed into a furnace to be melted, then synthetic resin, etc. constituting the battery are combusted to be removed, and further, most of iron is made into slag to be removed, then nickel is reduced to be made into an alloy with a part of iron and recovered as ferronickel. This method has advantages to use equipment of an existing smelter as it is and to save time and effort for the treatment, however, the recovered ferronickel is not suitable for any uses other than a stainless raw material, and in addition, most of cobalt and rare earth elements are distributed to slag and discarded, and also cobalt and rare earth elements which are distributed to ferronickel are treated as impurities. Because of this, from a viewpoint of effective utilization of rare cobalt and rare earth elements, this method is not desirable.
As another method, for example, as referred to in Japanese Unexamined Patent Publication No. 3918041, there has been proposed a method for recovering a metal from a spent nickel-metal hydride storage battery, the method comprising the steps of: forming an aqueous phase by dissolving a storage battery scrap with acid; separating rare earth metals from the aqueous phase as a double sulfate; then precipitating iron from the aqueous phase by raising pH; performing liquid-liquid extraction of a filtrate obtained after the iron precipitation by using an organic extractant to separate zinc, cadmium, manganese, aluminum, and residual iron and rare earth elements, wherein the extractant and the pH value is selected so that, after extraction, substantially only nickel and cobalt are dissolved into the aqueous phase to remain at the same atomic ratio as at the time of having existed inside the storage battery scrap; subsequently, precipitating a nickel/cobalt alloy from the aqueous phase; and finally using the nickel/cobalt alloy as a master alloy in order to manufacture a hydrogen storage alloy.
However, in this method, it is not easy that nickel and cobalt is electrodeposited as an alloy so as to have exactly the same ratio as in battery composition, and there is a possibility that, depending on a solution composition and an electrolytic condition, an composition of an electrodeposited alloy might change. Therefore, in order to accomplish an exact alloy composition, it takes extra time and effort to analyze an obtained alloy each time, and then to add a required amount of a component which is insufficient and to dissolve it again.
Furthermore, it is known that battery characteristics change depending on an alloy composition, and the alloy composition is changed by adding a new component in order to improve performance of a battery and kept improved, and therefore, a recovered nickel alloy and a recovered cobalt alloy are not necessarily suitable as a raw material of a battery material.
Thus, there was not found a process in which nickel and cobalt, and furthermore rare earths can be recovered from a spent nickel-metal hydride battery to be reused.
In view of such situation, the present invention aims to provide a method for reusably separating and recovering nickel, cobalt, and rare earth elements from positive and negative electrode active materials which constitute a nickel-metal hydride battery.